Butadiene is a versatile raw material used in the production of a wide variety of synthetic rubbers, polymer resins and chemical intermediates. The largest uses for butadiene are the production of styrene butadiene rubber and polybutadiene rubber, which are used mainly in tire products. Butadiene is also one of the components used in the manufacture of acrylonitrile-butadiene-styrene, styrene-butadiene copolymer latex, styrene-butadiene block copolymers and nitrile rubbers.
There is a growing demand for butadiene caused by the growth in tire demand as well as reduced natural rubber production. World butadiene consumption is forecasted to grow at an average rate of about 2%+ per year.
The major source of butadiene is as a byproduct in the steam cracking of naphtha and gas oil to make ethylene and propylene. Steam cracking is a process by which hydrocarbon molecules are exposed to very hot steam, causing them to break apart into smaller molecules. Separation of butadiene from the other products of the steam cracking process typically includes the use of extractive distillation.
Other potential sources for the production of butadiene include converting feed stocks comprising butene and butane compounds and mixtures thereof to butadiene. Isobutene has been used for the synthesis of MTBE. The market for MTBE, however, is decreasing, especially in the United States. Thus, there is emerging a relative abundance of isobutene. The various C4 streams represent alternative feed stocks for the production of butadiene. Unfortunately, industrial processes have not been developed or designed to effect efficient conversion and selectivity of butadiene from these sources, in particular when they contain a significant amount of isobutene and/or isobutane.
Various processes for butene isomerization are described in U.S. Pat. Nos. 3,531,545; 4,132,745; 5,157,194; and 6,743,958. The processes described in these patents are directed to butene isomerization reactions rather than production of butadiene.
“Reverse” isomerization of isobutene to n-butenes is described in Japanese Patent Application Nos. 2004-009136 and 2004-009138, and literature references Gon Seo et al., “The Reversible Skeletal Isomerization between n-Butenes and Iso-butene over Solid Acid Catalysts” Catalysis Today 44 (1998) 215-222, and Lucia M. Petkovic and Gustavo Larsen, “Linear Butenes from Isobutene over H-Ferrierite: In Situ Studies Using an Oscillating Balance Reactor”, J. of Catalysis 191, 1-11 (2000). These processes are not directed to production of butadiene.
U.S. Pat. No. 6,743,958 to Commereuc et al. describes an integrated process including the separate steps of: (1) selective hydrogenation of butadiene with isomerization of 1-butene into 2-butenes; (2) the skeletal (“reverse”) isomerization of isobutene into n-butenes; and (3) the metathesis of a 2-butene-rich fraction with ethylene. U.S. Pat. No. 5,157,194 to Rahmim et al. describes a method for the high level conversion of n-olefin-containing hydrocarbon streams to iso-olefin-rich product streams using a catalyst composition comprising microcrystalline ZSM-22.
Japanese Patent Application No. 2004-009136 describes isomerizing isobutene to n-butenes using ferrierite or γ-alumina. Japanese Patent Application No. 2004-009138 describes isomerizing isobutene to n-butenes using γ-alumina with water co-feed. U.S. Pat. Nos. 6,242,661 and 6,849,773 to Podrebarac et al., incorporated herein by reference in their entirety, describe the use of a combination butenes isomerization reaction and distillation tower to convert 1-butene to 2-butenes while fractionating to separate isobutene (and isobutane) from 2-butenes (and n-butane).
All of these references are generally directed to isomerization reactions or to utilizing the products for metathesis. None of these references include the dehydrogenation of C4 compounds such as n-butenes to butadiene.
Hydrocarbon Processing, November 1978, pp 131-136 by PetroTex, describes the oxydehydrogenation of n-butenes to butadiene. However, this reference does not describe butene isomerization, “reverse” isomerization, or methods to reduce or eliminate the disadvantageous impacts of isobutene by removing it. In addition, this reference does not describe the transformation of the unwanted isobutene into additional productive n-butenes and obtaining supplemental production of butadiene by adding conversion of isobutene. Moreover, the oxydehydrogenation process described in this article has high costs due to the use of a very large amount of steam to dilute the mixture and limit the reaction temperature rise in an adiabatic packed bed reactor.
U.S. Pat. Nos. 3,668,147; 4,547,615; and 7,034,195, describe the general production of butadiene. U.S. Pat. No. 7,034,195 to Schindler et al. describes an integrated process for preparing butadiene from n-butane via (1) feeding n-butane into a first dehydrogenation zone, autothermally (i.e., with some exothermic oxygen reaction, e.g., combustion, to balance heat requirement but not as a direct oxidative dehydrogenation reaction) converting n-butane to 1-butene, 2-butenes and optionally butadiene, (2) feeding the first product gas stream into a second dehydrogenation zone, which does oxidatively convert 1-butene and 2-butenes to butadiene.
U.S. Pat. No. 4,547,615 to Yamamoto describes oxidative dehydrogenation of monoolefin to a conjugated C4+ diolefin via a mixed metal oxide, with primary metals as Mo, Bi, Cr, Ni, etc. U.S. Pat. No. 3,668,147 to Yoshino et al. describes several reactions including butadiene production via mixed metal oxides, primarily Fe/Sb/V or Mo or W/Te/etc.
These references, however, do not describe industrial processes to efficiently and selectively produce butadiene from C4 feed stocks that contain a significant amount of isobutene and/or isobutane. Butadiene production processes from these feed stocks must address, among other issues, the undesirability of isobutene in the dehydrogenation step to butadiene and the nearly identical volatilities of isobutene and 1-butene making them essentially impossible to separate by standard distillation. Of the four butene species (cis-2-butene, trans-2-butene, 1-butene and isobutene), isobutene does not substantially form butadiene via dehydrogenation and in oxydehydrogenation is reactive towards direct combustion and formation of some amount of undesirable oxygenated and other byproducts. This also results in increased oxygen consumption. In addition, it causes catalyst deactivation. Consequently, it is undesirable to have a significant amount of isobutene in the dehydrogenation feed. If present at a substantial level, isobutene in the feed stocks must be separated from the n-butenes and n-butane.
However, it is very difficult to completely separate isobutene from all the n-butenes by distillation. In particular, isobutene and 1-butene are considered “co-boilers” because they differ by less than 1° C. in boiling points, at about −6° C. at atmospheric pressure. The 2-butenes boil at 1-4° C. Accordingly, elimination of 1-butene by isomerizing it to 2-butenes enables enhanced separation of isobutene from n-butenes by distillation in accordance with the processes of the present invention.
In addition to obtaining benefit by excluding isobutene from the feed to the nC4 dehydrogenation unit, an additional benefit can be obtained by converting the isobutene to additional n-butenes by “reverse” isomerization to augment the feed to the nC4 dehydrogenation unit. Isobutene/n-butene isomerization has historically focused on isobutene formation because of the demand for MTBE. Because n-butenes are not typically sold commercially, there has been little incentive for research on the “reverse” conversion of isobutene to n-butenes.
Isobutane also does not form butadiene via its direct dehydrogenation, though it is not harmful in terms of reacting significantly to undesired byproducts. On the other hand, extra butadiene production can be obtained from isobutane if it is dehydrogenated to isobutene and then the isobutene undergoes the “reverse” isomerization described above, creating additional n-butenes that can eventually be converted to butadiene. Thus, a unit to dehydrogenate isobutane to isobutene may be added for this purpose in accordance with this invention.
A different dehydrogenation unit to convert n-butane to n-butenes and possibly some amount of butadiene may also be added to the overall process plant in accordance with the present invention.
While many industrial processes have been investigated for, and are related to, the production of butadiene, none have been developed and designed for the conversion of C4 feed stocks containing a significant amount of isobutene and/or isobutane. As such, there exists an ongoing and unmet need in the industry for economical and efficient methods for butadiene production from these feed stocks.